## 2011 Ph.D Thesis Defenses

### Janine Dahl

Title:The Spectrum of the Off-diagonal Fibonacci Operator

Date:Tuesday, May 4, 2010

Thesis Advisor: David Damanik

Abstract:

The family of off-diagonal Fibonacci operators can be considered as Jacobi matrices acting in .e2(Z) with diagonal entries zero and off-diagonal entries given by sequences in the hull of the Fibonacci substitution sequence. The spectrum is independent of the sequence chosen and thus the same for all operators in the family. The spectrum is purely singular continuous and has Lebesgue measure zero. We will consider the trace map and its relation to the spectrum. Upper and lower bounds for the Hausdorff and lower box counting dimensions of the spectrum can be found under certain restrictions of the elements of the Fibonacci substitution sequence, and results from hyperbolic dynamics can be used to show that equality can be achieved between the two dimensions.

### Jon Fickenscher

Title: Self-Inverses in Rauzy Classes

Date: Tuesday, March 15, 2011

Thesis Advisor: William Veech

Abstract:

Thanks to works by M. Kontsevich and A. Zorich followed by C. Boissy, we have a classification of all Rauzy Classes of any given genus. It follows from these works that Rauzy Classes are closed under the operation of inverting the permutation. In this paper, we shall prove the existence of self-inverse permutations in every Rauzy Class by giving an explicit construction of such an element satisfying the sufficient conditions. As a corollary, we will give another proof that every Rauzy Class is closed under taking inverses. In the case of generalized permutations, generalized Rauzy Classes have been classified by works of M. Kontsevich, H. Masur and J. Smillie, E. Lanneau, and again C. Boissy. We state the definition of self-inverse for generalized permutations and prove a necessary and sufficient condition for a generalized Rauzy Class to contain self-inverse elements.

### Thomas McGaffey

Title: Regularity and Nearness Theorems for Families of Local Lie Groups

Date: Monday, April 11, 2011

Thesis Advisor: Robert Hardt

Abstract:

In this work, we prove three types of results with the strategy that, together, the author believes these should imply the local version of Hilbert's Fifth problem. In a separate development, we construct a nontrivial topology for rings of map germs on Euclidean spaces. First, we develop a framework for the theory of (local) nonstandard Lie groups and within that framework prove a nonstandard result that implies that a family of local Lie groups that converge in a pointwise sense must then differentiability converge, up to coordinate change, to an analytic local Lie group, see corollary 6.3.1. The second result essentially says that a pair of mappings that almost satisfy the properties defining a local Lie group must have a local Lie group nearby, see proposition 7.2.1. Pairing the above two results, we get the principal standard consequence of the above work which can be roughly described as follows. If we have pointwise equicontinuous family of mapping pairs (potential local Euclidean topological group structures), pointwise approximating a (possibly differentiably unbounded) family of differentiable (sufficiently approximate) almost groups, then the original family has, after appropriate coordinate change, a local Lie group as a limit point. (See corollary 7.2.1 for the exact statement.) The third set of results give nonstandard renditions of equicontinuity criteria for families of differentiable functions, see theorem 9.1.1. These results are critical in the proofs of the principal results of this paper as well as the standard interpretations of the main results here. Following this material, we have a long chapter constructing a Hausdorff topology on the ring of real valued map germs on Euclidean space. This topology has good properties with respect to convergence and composition. See the detailed introduction to this chapter for the motivation and description of this topology.

### Shuijing Li

Title: Rational points on del Pezzo surfaces of degree 1 and 2

Date: Thursday, June 10, 2010

Thesis Advisor: Brendan Hassett

Abstract:

One of the fundamental problems in Algebraic Geometry is to study solutions to certain systems of polynomial equations in several variables, or in other words, find rational points on a given variety which is defined by equations. In this paper, we discuss the existence of del Pezzo surface of degree 1 and 2 with a unique rational point over any finite field [Special characters omitted.] , and we will give a lower bound on the number of rational points to each q. Furthermore, we will give explicit equations of del Pezzo surfaces with a unique rational point. Also we will discuss the rationality property of the del Pezzo surfaces especially in lower degrees.

### Carolyn Otto

Title: The (n)-Solvable Filtration of the Link Concordance Group and Milnor's mu-Invariaants

Date: Friday, April 8, 2011

Thesis Advisor: Shelly Harvey

Abstract:

We establish several new results about the ( n )-solvable filtration, [Special characters omitted.] , of the string link concordance group [Special characters omitted.] . We first establish a relationship between ( n )-solvability of a link and its Milnor's μ-invariants. We study the effects of the Bing doubling operator on ( n )-solvability. Using this results, we show that the "other half" of the filtration, namely [Special characters omitted.] , is nontrivial and contains an infinite cyclic subgroup for links with sufficiently many components. We will also show that links modulo (1)-solvability is a nonabelian group. Lastly, we prove that the Grope filtration, [Special characters omitted.] of [Special characters omitted.] is not the same as the ( n )-solvable filtration.

### Ryan Scott

Title: Minimizing the mass of the codimension-two skeleton of a convex, volume-one polyhedral region

Date: Wednesday, April 13, 2011

Thesis Advisor: Robert Hardt

Abstract:

In this paper we establish the existence and partial regularity of a (d-2)-dimensional edge-length minimizing polyhedron in [Special characters omitted.] . The minimizer is a generalized convex polytope of volume one which is the limit of a minimizing sequence of polytopes converging in the Hausdorff metric. We show that the (d-2)-dimensional edge-length ζ d -2 is lower-semicontinuous under this sequential convergence. Here the edge set of the limit generalized polytope is a closed subset of the boundary whose complement in the boundary consists of countably many relatively open planar regions.